The invention relates to the general field of imaging, and more particularly to three-dimensional images (or 3D images) of parts (i.e. pieces) made of composite materials (e.g. parts made of carbon fibers and a carbon matrix or resin), such as for example images of composite material parts obtained by computed tomography.
A preferred but non-limiting application of the invention lies in numerous branches of engineering industry, and in particular in the aviation industry.
It is known at present to perform non-destructive inspection of composite material parts by using 3D images obtained by computed tomography, in particular to inspect the health of the material (i.e. absence of defects) of such parts.
Computed tomography is an imaging technique that makes use of a succession of photographs taken of a composite material part in order to reconstruct a three-dimensional image of the part that is encoded on a gray scale. The gray-scale level of a voxel in the 3D image is proportional to the density of the material of the part in that voxel. By using suitable image-analysis techniques, the 3D image makes it possible to detect automatically defects, if any, affecting the inspected material part.
Nevertheless, certain image-analysis methods are highly dependent on the quality of the 3D tomographic image under consideration.
In the state of the art, there exist standards that describe methods for measuring the quality of a tomographic image. By way of example, such standards include:                the E1441-00RT standard of the American Society for Testing and Materials (ASTM), as described in the document entitled “Standard guide for computed tomography imaging”, which defines using computed tomography for non-destructive inspection and tomography quality metrics; and        the equivalent European standard NF EN 16016-3 described in greater detail in the document entitled “Essais non destructifs— Méthodes par rayonnements—Tomographie informatiisée Partie 3: Fonctionnement et interprétation” [Non-destructive testing—radiation methods—computed tomography—part 3: operation and interpretation].        
Those standards propose specifically the following quality metrics or indices:                a standard deviation of noise measured over a uniform zone of the 3D image that is not deformed by artifacts;        a signal-to-noise ratio defined as being the ratio of the mean gray-scale level of the material part under consideration divided by the standard deviation of the noise; and        a contrast-over-noise ratio defined as being the ratio of the absolute value of the difference between the mean gray-scale level of the part under investigation and the mean gray-scale level of the background of the 3D image divided by the standard deviation of the noise.        
Nevertheless, those metrics are designed for a part made of a single material, typically for a metal part.
Unfortunately, with a composite material, it is not possible to find in the 3D image a homogeneous zone, i.e. made up of a single phase of material, that is sufficiently representative (i.e. large enough to be statistically viable), since the fibers and the resin are intimately intermingled by construction in a composite material.
Consequently, if it is desired to use such metrics for estimating the quality of a 3D image of a composite material part, it is necessary to add reference bars that are themselves made of a single material in the setup under consideration while acquiring the 3D image so as to enable such metrics to be estimated.
Even when such a constraint is satisfied, it still remains that the metrics that are obtained involve only one phase of material (and possibly also the background of the 3D image, depending on the metrics under consideration), and are therefore incomplete for characterizing the quality of a 3D image of a part made of composite material.